1. Field of the Invention
The present invention relates to a heatsink and a fabrication method thereof. Particularly, the present invention relates to a heatsink on which is mounted a semiconductor element of relatively great heat generation such as a laser diode, a CPU (central processing unit), a MPU (microprocessor unit), a high frequency amplifier device, and the like, having a multilayer structure of a diamond layer and a metal layer, and a method of fabricating such a heatsink.
2. Description of the Background Art
The aforementioned high power semiconductor devices generate a great amount of heat during operation. The heat generated by these semiconductor elements has become greater in accordance with improvement in the output and the operating frequency. The need of compact and light-weight electronic equipment is great in the industry while the packaging density of the semiconductor element is continuously increasing. The increase in the heat generation and packaging density of semiconductor elements implies more stringent requirements with respect to the heat radiation characteristic of the heatsink employed in modules in which high power semiconductor elements are mounted.
Regarding such heatsinks that require great heat dissipation, a semiconductor element is mounted on a heatsink formed of a material of high thermal conductivity to prevent the semiconductor element from becoming too hot. For heatsinks that incorporate high power semiconductor elements such as a high power transistor or microwave monothylic IC (MMIC) of great heat generation, beryllium oxide (BeO), for example, superior in thermal conductivity and dielectric property is conventionally used widely.
Diamond is known as the substance having the highest thermal conductivity. Research has been effected to apply diamond to the heatsink that is used for incorporating a semiconductor element.
As a heatsink employing diamond, development is in progress of a heatsink formed entirely of diamond, and a heatsink having a diamond film formed on a metal substrate.
Since natural diamond is precious and artificial diamond is costly, the cost of the heatsink will increase if the amount of diamond therein becomes greater. Therefore, a heatsink formed entirely of diamond is used with respect to a semiconductor element of high heat generation such as a high power laser only in the application where heat radiation is so insufficient that it prevents exhibition of proper performance when a substitute is used or in the application such as during the stage of research where the cost is not yet estimated. A heatsink having a diamond film formed on a metal substrate is used in products that must have the cost reduced.
By using a heatsink formed partially of metal, the cost can be decreased although the thermal conductivity is degraded in comparison to a heatsink formed only of diamond. Therefore, the cost and performance of the heatsink is substantially proportional. It can be said that a heatsink of higher thermal conductivity becomes more expensive. Therefore, there is a demand for an economical heatsink of high thermal conductivity.
In response to such demands, a heatsink of a multilayered structure with a thin diamond film formed on a metal of favorable thermal conductivity is disclosed in, for example, Japanese Patent Laying-Open No. 5-326767.
Conventionally, BeO superior in thermal conductivity has been widely used for the heatsink. However, the level of the heat radiation characteristic that is currently required has become so high that even BeO is even not sufficient. An approach has been made to reduce the thickness of the BeO substrate to reduce thermal resistance. However, it is difficult to process BeO per se. Furthermore, BeO is toxic. It can be said that reduction in the thickness has come to its limit.
As to the heatsink disclosed in the above publication, copper and copper-tungsten alloy which are metals of favorable thermal conductivity are mentioned as the substance of the substrate. These materials are suitable for the heatsink since the thermal conductivity thereof is high comparable to other metal materials and the cost is relatively low.
However, there was problem that it is difficult to grow a thin diamond film on a substrate that includes copper in favorable adherence since the copper in the substrate does not produce carbide, does not absorb carbon, and is not occluded with carbon, as described in New Diamond, Vol. 10, No. 3 (34), pp 26 and 27
Copper has a high thermal expansion coefficient whereas diamond has a low thermal expansion coefficient. Therefore, there is a problem that the thin diamond film will peel off the substrate as the temperature of the heatsink becomes higher due to the difference in thermal expansion coefficient between copper and diamond.
If the difference in thermal expansion between the substrate and the diamond is small, warping in the diamond heatsink will not occur. Only stress will be generated within the thin diamond film even when the heatsink attains high temperature. However, the thermal expansion of copper or a sintered compact including copper is greater than that of diamond, resulting in the problem of warping in the heatsink.
In view of the foregoing, an object of the present invention is to provide a heatsink that can have a thin diamond film formed in good adherence on a substrate of favorable thermal conductivity.
Another object of the present invention is to provide a heatsink that can have occurrence of warping suppressed.
According to an aspect of the present invention, a heatsink includes a substrate of a sintered compact including Cu and W, and a thin diamond film layer formed on the surface of the substrate. The Cu content in the substrate is at least 5% by weight. In an X-ray diffraction chart obtained by irradiating a thin diamond film layer with an X-ray, the diffraction peak intensity of the (110) plane of W is at least 100 times the diffraction peak intensity of the (200) plane of Cu.
In such a heatsink, the amount of W at the surface of the substrate is relatively great whereas the amount of Cu at the surface of the substrate becomes relatively smaller. Therefore, the adherence between the substrate and the thin diamond film layer formed on the surface of the substrate is improved. As a result, the heat locally generated from the semiconductor element mounted on the thin diamond film layer can be promptly dissipated at the in-plane of the thin diamond film layer by virtue of the effect of the thin diamond film layer as a heat spreader (effect of heat dissipation) to be conveyed to the substrate. The thermal conductivity of the substrate is increased since the Cu content in the substrate is at least 5% by weight.
In an X-ray diffraction chart obtained by irradiating the thin diamond film layer with an X-ray, it is preferable that a peak of WC (tungsten carbide) appears. In this case, the adherence between the thin diamond film layer and the substrate is improved.
According to another aspect of the present invention, a heatsink includes a substrate of a sintered compact including Cu and W, and a thin diamond film layer formed on the surface of the substrate. The Cu content in the substrate is at least 5% by weight. In an X-ray diffraction chart obtained by irradiating the thin diamond film layer with an X-ray, the diffraction peak intensity of the (211) plane of W is at least 30 times the diffraction peak intensity of the (200) plane of the Cu.
In such a heatsink, the amount of W at the surface of the substrate becomes relatively greater whereas the amount of Cu at the surface of the substrate becomes relatively smaller. Therefore, the adherence between the substrate and the thin diamond film layer formed on the surface of the substrate is improved. As a result, the heat locally generated from the semiconductor element mounted on the thin diamond film layer is rapidly dissipated at the in-plane of the thin diamond film layer to be subsequently conveyed to the substrate by virtue of the effect of the thin diamond film layer as a heat spreader (effect of heat dissipation). Also, the thermal conductivity of the substrate becomes higher since the Cu content in the substrate is at least 5% by weight.
In an X-ray diffraction chart obtained by irradiating the thin diamond film layer with an X-ray, it is preferable that a peak of WC (tungsten carbide) appears. In this case, the adherence between the thin diamond film layer and the substrate is improved.
According to a further aspect of the present invention, a heatsink includes a substrate including Cu and a metal of a low thermal expansion coefficient, and a thin diamond film layer formed on the surface of the substrate. The Cu content in the substrate is at least 5% by weight. The Cu content in the substrate becomes lower as a function of approaching the surface of the substrate.
In the heatsink of the above structure, the Cu content at the surface of the substrate is lowest. Therefore, the adherence between the substrate and the thin diamond film layer on the substrate is improved since the amount of Cu that does not easily attach to carbon is small at the surface of the substrate. As a result, the heat locally generated from the semiconductor element is rapidly dissipated at the in-plane of the thin diamond film layer to be subsequently conveyed to the substrate by virtue of the thin diamond film layer serving as a heatsink spreader (effect of heat dissipation). Also, the thermal conductivity of the substrate is increased since the Cu content in the substrate is at least 5% by weight.
The Cu content at a region of the substrate that is not more than 10 xcexcm in depth from the surface of the substrate is preferably not more than 50% of the entire Cu content of the substrate. By adjusting the content amount of Cu at the surface of the substrate, adherence between the thin diamond film layer and the substrate is improved. If the Cu content at a region that is not more than 10 xcexcm in depth from the surface of the substrate exceeds 50% of the entire Cu content of the substrate, the rate of presence of Cu becomes so high that the thin diamond film layer is easily peeled off the substrate. By setting the Cu content at the surface of the substrate to be less than 50% of the entire Cu content of the substrate, warping in the substrate can be suppressed due to an appropriate amount of Cu remaining in the substrate.
The substrate is preferably a Cuxe2x80x94W sintered compact or a Cuxe2x80x94Wxe2x80x94Mo sintered compact. The Cuxe2x80x94W sintered compact or the Cuxe2x80x94Wxe2x80x94Mo sintered compact must have a thermal conductivity of at least 100 W/mxc2x7K in order to exhibit the advantage of the present invention.
W particles are exposed at the surface of the substrate. The surface roughness RZ of the W particle is preferably at least 0.05 xcexcm. The diamond nucleus is easily generated from the convex portion of the W particle. By setting the surface roughness RZ of the W particle as above, the nucleus generation density of diamond is improved. Therefore, the number of contact points between the substrate and the thin diamond film layer is increased. Thus, adherence between the thin diamond film layer and the substrate can further be improved.
If the surface roughness RZ of the W particle is less than 0.05 xcexcm, the nucleus generation density is reduced since the convex portion in the W particle is reduced. This means that the adherence between the thin diamond film layer and the substrate is degraded so that the thin diamond film layer easily peels off the substrate.
Preferably, an intermediate layer in which the Cu content is approximately 0% by weight is formed between the surface of the substrate and the thin diamond film layer. By provision of an intermediate layer that does not include Cu between the substrate and the thin diamond film layer, the thin diamond film layer will not come into contact with the Cu in the substrate. Therefore, adherence between the thin diamond film layer and the substrate is further improved.
The thickness of the substrate is preferably at least 200 xcexcm and not more than 10000 xcexcm. In order to maintain the strength as a substrate, the thickness of the substrate is preferably at least 200 xcexcm. In order to avoid the thermal resistance of the heatsink from becoming too great, the substrate thickness is preferably not more than 10000 xcexcm.
The thickness of the thin diamond film layer is preferably at least 10 xcexcm. In this case, the thin diamond film layer functions to diffuse in-plane the heat generated by the semiconductor element to prevent the heat from being partially confined. The thickness of the thin diamond film layer must be at least 10 xcexcm for this function.
The thermal conductivity of a thin diamond film layer is generally in the range of 500 W/mxc2x7K-2000 W/mxc2x7K depending upon the quality of the diamond. In the present invention, the thermal conductivity of the thin diamond film layer must be at least 700 W/mxc2x7K in order to exhibit the effect of the present invention.
Preferably, the substrate has a thermal conductivity of at least 100 W/mxc2x7K, and a thickness of at least 200 xcexcm and not more than 700 xcexcm, whereas the thin diamond film layer has a thickness of at least 10 xcexcm and not more than 200 xcexcm. Here, the thin diamond film layer expands analogous to the substrate since it is very thin. Therefore, the expansion of the thin diamond film layer and that of the semiconductor element provided thereon is substantially equal. Accordingly, cracking in the semiconductor element can be suppressed.
Furthermore, the thermal conductivity of the thin diamond film layer is preferably at least 100 W/mxc2x7K.
A method of fabricating a heatsink according to the present invention includes the steps of reducing the Cu content at the surface layer region of the substrate by immersing the surface of the substrate including Cu and a metal of a low thermal expansion coefficient in acid, and roughening the surface of the exposed metal of a low thermal expansion coefficient, and forming by vapor synthesis a thin diamond film layer on the surface of the substrate subjected to acid treatment.
According to the heatsink fabrication method including the above steps, the Cu content at the surface of the substrate is reduced by subjecting the surface of the substrate to acid treatment. Then, a thin diamond film layer is formed on the surface. Therefore, a thin diamond film layer can be formed on the substrate in good adherence.
The step of reducing the Cu content at the surface layer region of the substrate includes roughening the exposed surface of the metal of a low thermal expansion coefficient. Therefore, adherence between the thin diamond film layer and the substrate is further improved since the thin diamond film is formed on the roughened metal of low thermal expansion coefficient.
The acid is preferably a solution selected from the group consisting of hydrochloric acid, nitric acid, sulfuric acid, hydrofluoric acid (HF), hydrogen peroxide (H2O2) and chromic acid, or a mixed solution thereof. By using these acids, the surface of the substrate can be appropriately roughened to facilitate formation of the thin diamond film layer.
The step of reducing the Cu content at the surface layer region of the substrate preferably includes a first acid treatment step of immersing the surface of the substrate in a certain acid, and a second acid treatment step of immersing the substrate subjected to the first acid treatment in an acid different from the certain acid.
In forming a diamond film by vapor synthesis, a diamond nucleus is generated at a deep location from the surface of the substrate. In other words, the root of the thin diamond film is present at a deep region in the substrate, so that the anchor effect can be expected.
The substrate is preferably at least one type of a sintered compact selected from the group consisting of a Cuxe2x80x94W sintered compact, a Cuxe2x80x94Mo sintered compact and a Cuxe2x80x94Wxe2x80x94Mo sintered compact.
W particles are exposed at the surface of the substrate immersed in acid. The surface roughness RZ of the W particle is preferably at least 0.05 xcexcm. By such a surface roughness, small particles of diamond permeate into the surface of the W particles. A thin diamond film layer is grown with the diamond particle as the nucleus. Thus, adherence between the thin diamond film layer and the substrate is improved.
The acid treatment that reduces the Cu content is preferably carried out until the peak of Cu is not detected in an X-ray diffraction chart obtained by irradiating the surface of the substrate with an X-ray.
In this case, the Cu content at the surface of the substrate is sufficiently reduced. Therefore, adherence between the substrate and the thin diamond film layer formed thereon is improved.
The acid treatment of reducing the Cu content is preferably carried out until the porosity of the region of the substrate that is within 30 xcexcm in depth from the surface of the substrate is at least 5% by volume and not more than 70% by volume, and the Cu content at the region within 30 xcexcm in depth from the surface of the substrate is not more than 50% of the entire Cu content of the substrate.
By setting the porosity to the above described range, the grains of diamond can easily permeate into the hole to facilitate nucleus generation.
If the porosity is less than 5% by volume, nucleus generation cannot easily occur. If the porosity is greater than 70% by volume, the holes become so great that the thermal conductance is reduced. As a result, the property as a heatsink will be degraded.
Further preferably, the porosity at the region within 30 xcexcm in depth from the surface of the substrate is at least 10% by volume and not more than 50% by volume.
By setting the Cu content at the region of the substrate within 30 xcexcm in depth from the surface of the substrate to be less than 50% of the entire Cu content of the substrate, warping of this heatsink will not easily occur.
Preferably, a step of carrying out a process of scratching the surface of the substrate is included prior to formation of the thin diamond film layer. In this case, diamond is easily nucleated from the scratch at the surface of the substrate. Many diamond nuclei can be generated at the surface of the substrate to promote the speed in the growth of the thin diamond film layer. Also, the thickness of the thin diamond film layer can be made uniform.
The scratching process preferably includes the step of scratching the surface of the substrate using diamond. In this case, diamond is attached to the surface of the substrate in the scratching step of the surface of the substrate to become the nucleus in forming the thin diamond film layer. Therefore, growth of the thin diamond film layer can further be promoted.
Vapor synthesis is used in forming a thin diamond film layer on the substrate. This vapor synthesis includes hot filament CVD (chemical vapor deposition), plasma CVD, flame method, and the like.
According to still another aspect of the present invention, a heatsink includes a substrate, and a thin diamond film layer formed on the substrate. The substrate includes a porous body of a low thermal expansion coefficient, and Cu filled in the hole of the porous body. At the surface layer of the substrate, a thin diamond film layer is present in the hole. The diamond film layer is permeating into the hole at a surface layer of the porous body.
In the heatsink of the above structure, the difference in thermal expansion coefficient between the substrate and the thin diamond film layer formed on the substrate is small since the porous body forming the substrate has a low thermal expansion coefficient. Adherence between the substrate and the thin diamond film layer is improved since the thin diamond film layer is present at the surface layer of the porous body. As a result, the thin diamond film layer can be prevented from peeling off the substrate even when the heatsink attains high temperature.
Preferably, the substrate has a thermal conductivity of at least 10 W/mxc2x7K and a thickness of at least 200 xcexcm and not more than 700 xcexcm whereas the thin diamond film layer has a thickness of at least 10 xcexcm and not more than 200 xcexcm. The thin diamond film layer expands substantially analogous to the substrate due to its small thickness. Therefore, the expansion of the thin diamond film layer is substantially equal to that of the semiconductor element provided thereon. As a result, cracking in the semiconductor element can be suppressed.
Furthermore, the thermal conductivity of the thin diamond film layer is preferably at least 1000 W/mxc2x7K.
The porous body is preferably at least one type of a sintered compact selected from the group consisting of a W sintered compact, a Mo sintered compact, and a Wxe2x80x94Mo sintered compact.
Furthermore, the porosity of the porous body is preferably at least 15% by volume and not more than 60% by volume. If the porosity of the porous body is less than 15% by volume, the heat conductivity is reduced when the hole is filled with Cu. If the porosity exceeds 60% by volume, the thickness of the thin diamond film layer will become nonuniform.
A heatsink fabricating method according to the present invention includes the steps of forming a thin diamond film layer on the surface of a porous body having a low thermal expansion coefficient, and filing the hole of the porous body with Cu after formation of the thin diamond film layer.
According to the heatsink fabrication method including the above steps, a diamond nucleus is generated from the surface of the porous body in forming a thin diamond film layer. Therefore, the nucleus of the thin diamond film layer is generated from a deep portion remote from the surface of the porous body. In other words, the base of the thin diamond film layer is present at a deep region remote from the surface of the substrate. Therefore, the anchor effect can be expected.
Since a substrate of a porous body with a low thermal expansion coefficient is used, the amount of thermal expansion is small when a thin diamond film layer is grown. Since the amount of thermal expansion is small, warping that occurs by the difference in thermal expansion between the thin diamond film layer and the porous body can be suppressed during the stage of cooling the porous body after film growth.
Since Cu fills the hole of the porous body, the hole of the porous body is exactly filled with Cu that has favorable thermal conductivity. As a result, the thermal conductance of the substrate is improved. Accordingly, the thermal conductance of the entire heatsink is improved.
Since the porous body is absent of Cu during the stage of forming a thin diamond film layer on the porous body, the thin diamond film layer can be formed in favorable adherence on the porous body.
The step of forming a thin diamond film layer preferably includes the step of forming a thin diamond film layer on the surface of the porous body by vapor synthesis. Here, the vapor synthesis method includes the hot filament CVD (chemical vapor deposition), plasma CVD, flame method, and the like.
The porous body is preferably at least one type of a sintered compact selected from the group consisting of a W sintered compact, a Mo sintered compact, and Wxe2x80x94Mo sintered compact.
The porosity of the porous body is preferably at least 15% by volume and not more than 60% by volume. By setting the porosity to the above range, generation of a diamond nucleus from the surface of the porous body is facilitated. The thermal conductivity when Cu is filled is increased to improve the thermal conductivity of the entire sink.
If the porosity is less than 15% by volume, a diamond nucleus cannot be easily generated from a deep region in the porous body. As a result, adherence between the porous body and the thin diamond film layer is degraded. Furthermore, the thermal conductance of the heatsink is degraded since the amount of Cu in the hole is reduced.
If the porosity exceeds 60% by volume, it will become difficult to form a thin diamond film layer of uniform thickness on the surface of the porous body although a diamond nucleus can be generated at a deep region of the porous body. Furthermore, although the generation density of the diamond nucleus is reduced and the grain of the diamond forming the thin diamond film layer is increased, the surface roughness of the thin diamond film layer becomes greater. Accordingly, there is a problem that the thickness of the thin diamond film layer is not uniform, and polishing the thin diamond film layer is time consuming.
The step of filling the hole of the porous body with Cu preferably includes the step of permeating molten Cu into the hole of the porous body.
The step of filling the hole of the porous body with Cu preferably includes the step of heating and melting Cu and permeating molten Cu in the hole after the porous body is placed on solid Cu.
By placing solid Cu on a heating device such as a heater, arranging the porous body thereon with a thin diamond film layer at the top surface and the Cu melted, the Cu will permeate into the porous body by the capillary action. According to this method, arrangement between Cu and the porous body is feasible. Furthermore, since the molten Cu can be suppressed from spattering around, adherence of contaminants on the surface of the thin diamond film layer can be prevented.
The step of filling the hole of the porous body with Cu preferably includes the step of heating and melting Cu to permeate the molten Cu into the hole after solid Cu is placed on the porous body where the thin diamond film layer is formed.
Since the solid Cu placed on the porous body is melted, the Cu permeates into the porous body by the weight and capillary action of the Cu. Therefore, the charging rate of Cu becomes faster.
The step of filling the hole of the porous body with Cu preferably includes the step of storing molten Cu in a container and immersing the porous body with the formed thin diamond film layer in the molten Cu to permeate the melted Cu into the hole.
Since the porous body is immersed in the molten Cu, Cu permeates equally from all the faces of the porous body except for the face where the thin diamond film layer is formed. Also, the permeation speed becomes faster.
Preferably, the step of scratching the surface of the porous body prior to formation of a thin diamond film layer is included. Since a diamond nucleus is easily generated from the scratch, many diamond nucleus are generated at the surface of the porous body. A thin diamond film layer is grown faster and with uniform thickness.
The scratching process preferably includes the step of scratching the surface of the porous body using diamond. Diamond is attached to the surface of the porous body during the stage of scratching the surface of the porous body. This diamond becomes the nucleus in forming a thin diamond film layer. Accordingly, the growth of the thin diamond film layer is facilitated.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.